Ultra-fine alumina (aluminum oxide) powder is one of the most widely used ceramic materials in a variety of industries. Applications of fine alumina powders include use as abrasives for polishing semiconductor and precision optical components, catalyst supports including the support structure in automobile catalytic converters, fillers for polymers, and pigment for painting, and the like. Alumina has over twelve (12) different crystalline phases, each of which has a different lattice structure and physical properties. However, the most well known and commonly used alumina powders are γ-alumina and α-alumina. The low temperature phase, γ-alumina, is thermodynamically metastable and transforms to the thermodynamically stable phase, α-alumina, at temperatures in excess of about 1100° C. or about 1200° C. depending on various conditions. With a defective spinel structure, γ-alumina powder can have very small particle sizes, e.g., particle sizes of less than about 20 nm, and extremely high surface area, e.g., greater than about 300 m2/g. Moreover, γ-alumina can be processed via both vapor and liquid phase processing techniques. Ultrafine γ-alumina having an average particle size of less than 40 nm and a polishing slurry with γ-alumina are commercially available.
The density of α-alumina is about 20% higher than the density of γ-alumina and more chemically and mechanically durable than γ-alumina. Thus, nanosized α-alumina particles should be suitable for a greater range of applications than nanosized γ-alumina. However, during the phase transformation, due to the reorganization of oxygen in the crystal lattice, the alumina particle size increases drastically such that α-alumina prepared from γ-alumina normally has a particle size of greater than 100 nm.
To make nanosized α-alumina, e.g., α-alumina particles of less than about 100 nm, has been a challenge for an extended period of time. To prevent the particle from rapid grain growth is the key. It is well known that fine α-alumina powders having an average particle size of greater than 100 nm can be prepared via a seeded sol-gel-process. In the process, boehmite is first peptized in acidic aqueous solution, containing nitric acid or acetic acid and then a couple of weight percent of α-alumina seeds, usually fine α-alumina particles, are added to the solution during the peptization to allow phase transformation to occur at lower temperature. The sol is oven dried at about 100° C. and converted to a dry gel. After crushing to micron sized granules, they are fired at a high temperature, normally over about 1000° C. to the produce of α-alumina particles. The temperature must be well controlled to prevent particle growth. However, in this process micron sized grains remain intact during the phase transformation process and result in mechanically strong hard grains of α-alumina after completion of the transformation. To make nanoalumina particles, high mechanical energy is required to crush or break down the grain into primary particles which typically have an average particle size of more than 100 nm. Moreover, the grinding process frequently results in high levels of impurity contamination.
U.S. Pat. No. 5,312,791 recites a modified approach to prepare alumina grains and fibers. The starting material is boehmite that is peptized and then dispersed in water to generate an alumina sol. The sol is rapidly cooled in liquid nitrogen or, alternatively slowly cooled by freeze drying. Water is sublimed under vacuum from the sol to form a gel composed of flakes having a thickness of between 1 and 3 μm. By the process recited in '791 patent, finer aluminal powders, flakes, fibers, and grains can be made having micron-sized smallest dimensions. However, as the powders themselves have no porosity, they require high mechanical energy grinding to form smaller particles which introduces high levels of impurities into the α-alumina product.
Methods of making α-alumina particles or grains using Seeded Gel (SG) technology via controlled solution chemistry has been known for several decades. Typically the process includes peptization of boehmite powder (AlOOH) in water by the addition of an acid, such as nitric acid, hydrochloric acid, or acetic acid, to form a uniform boehmite sol. Alumina seeds which are frequently fine α-alumina particles, which have been dispersed in water, are added to the boehmite sol and the admixture is combined thoroughly. The sol solution is then subjected to a drying process to transform the sol into a dried gel which is then pulverized and fired to a temperature at which point transformation to α-alumina is completed with minimal sintering. Because of the presence of the seed particles, transformation temperature is decreased from about 1200 to about 1250° C. for unseeded sols to about 1000 to about 1050° C. The α-alumina thus prepared can have sub micron particle sizes. However, no α-alumina particles prepared to date have an average particle size of less than about 100 nm.